1. Technical Field
The invention concerns an apparatus for and a method of determining distance. Apparatuses for and methods of determining distance in the sense of the present invention are based on the principle of emitting a light pulse and measuring the transit time between the commencement of emission of the light pulse and the reception of the components of the light pulse, which are reflected by an object. The distance to a reflecting object is afforded in that case in the form of the product of half the measured transit time and the speed of light.
2. Discussion of Related Art
Apparatuses for determining distance which are known in the state of the art have a transmitting unit for emitting a light pulse, a receiver matrix having at least one photoelectric element and a control unit which is connected to and controls the transmitting unit and the receiver matrix. The transmitting unit is adapted to emit a light pulse when an activation signal is applied to a first control input, in which case the light of the light pulse can come from the visible spectrum or other spectral ranges of electromagnetic radiation. Thus for example invisible light in the infrared frequency range is suitable for use in an apparatus for determining distance.
The receiver matrix has a measurement output which supplies an output signal. The output signal is derived from a measurement signal which is generated by the at least one photoelectric element of the receiver matrix in dependence on light incident thereon. Advantageously the transmitting unit and the receiver matrix are so selected that the photoelectric element of the receiver matrix reacts particularly strongly to light of the wavelength of the light pulse emitted from the transmitting unit so that the output signal produced by the receiver matrix exhibits a particularly strong dependency on the intensity of received light of that wavelength.
WO 99/34235 discloses a method of and an apparatus for recording a three-dimensional distance image which function in accordance with the outlined basic principle of what is known as the time-of-flight methods. In that case a light pulse of a given duration in emitted and at the same time the procedure begins to integrate the photoelectric current of a photoelectric element over the given duration of emission of the light pulse. Integration of the photoelectric current is also concluded at the same time with the end of emission of the light pulse. As the photoelectric element outputs a substantially higher photoelectric current from the moment in time from which reflected components of the emitted light pulse reach the photoelectric element and that photoelectric current is integrated until the end of the emission of the light pulse, the integrator state at the end of the measurement period gives information as to the delay with which (that is to say after what transit time) the reflected light pulse reached the photoelectric element and thus gives information as to the magnitude of the distance to the reflecting object.
More specifically there is only ever a first portion of the received reflected component of the emitted light pulse that contributes to the integrator state as the integration time ends before the reflected light pulse has been completely incident on the photoelectric element. The first portion of the received reflected component of the emitted light pulse, which portion is detected by the photoelectric element and integrated, is in that case correspondingly greater, the shorter the distance between the object and the photoelectric element. Distance measurement based on the time-of-flight method is therefore substantially based on the fact that only a portion of the received reflected component of the emitted light pulse is integrated.
To improve the level of measurement accuracy, it is proposed in the above-specified source that measurement of the dark current and the ambient light (background) is additionally implemented, in which case no light pulse is emitted and the integration result thus reflects solely the component of the photoelectric current caused by the ambient light, over the measurement period. So that in addition the measurement result is also made independent of the reflection coefficient of the reflecting object, two measurements are implemented involving integration times of differing lengths and the respective measurement results are standardized by subtraction and quotient formation.
All known methods of determining distance by measurement of the transit time of a light pulse suffer from the disadvantage that the intensity of the reflected components of the emitted light pulse decreases in square relationship with the distance to the reflecting object. As a result the signal-noise ratio in respect of determining distance worsens with increasing distance to the reflecting object. A further disadvantage of the known distance measurement methods is that in principle only a part of the reflected light pulse is integrated for distance determination purposes, the magnitude of the integrated component being dependent on the transit time of the light pulse. That means that the signal-to-noise ratio additionally worsens for the measurement procedure because a smaller integrated useful signal is confronted with a constant noise signal.